The present invention relates to a method for polishing a metal film, and particularly to a polishing method in the process of forming inlaid interconnect of semiconductor devices.
In view of the recent tendency to increase the degree of integration of semiconductor integrated circuits (hereinafter, referred to as LSI) and to improve the performance thereof, new techniques of micro fabrication are being developed. The chemical mechanical polishing method is one of these new techniques, and frequently utilized for the planarization of interlayer dielectric film, the formation of metal plugs and the formation of inlaid interconnect in the production of LSI and especially in the multilevel metallization fabrication process. This technique is disclosed in, for example, U.S. Pat. No. 4,944,836.
In the recent years, there is an attempt to replace the prior art aluminum alloys used as the wiring material by copper alloy of low resistance in order to improve the high-speed performance of LSI. However, micro fabrication of copper alloys is difficult to carry out according to the dry etching method, though it has frequently been used in the formation of aluminum alloy wiring. Thus, the so-called damascene method, which is a method of forming an inlaid interconnect by depositing thin films of copper alloy on a grooved insulating film and removing the copper alloy thin film by the CMP method only from the portions not inlaid in the grooves, is predominantly adopted. This technique is disclosed in, for example, JP-A 2-278822. Usually, TiN film, Ta film, TaN film or the like having a thickness of several tens nm are inserted between the insulating film and the copper alloy thin film for the purpose of improving the adhesion and forming a copper diffusion barrier.
Further, for formation of metal plug, tungsten is sometimes used according to the chemical vapor deposition (CVD) method because of its good inlaid characteristics. In this case, too, plug is mostly formed by CMP similarly to the above case, even though the etchback method is also usable.
The polishing slurries used for CMP of metal films such as copper alloy films, etc. generally contain an abrasive and an oxidizer (oxidizer is sometimes called oxidant) as main ingredients thereof. The fundamental mechanism of CMP consists in oxidizing the metal surface by the oxidizing action of oxidizer while mechanically removing the resulting oxide with the abrasive. This mechanism is disclosed in xe2x80x9cThe Science of CMPxe2x80x9d, edited by Masahiro Kashiwagi, published by Science Forum K. K., Aug. 20 (1997), page 299.
As the solid abrasive, alumina abrasive and silica abrasive having a particle size of several tens to several hundreds nm are known. Of these abrasives, most of the commercially available ones used for polishing metals belong to the alumina abrasive.
As the oxidizer, hydrogen peroxide (H2O2), ferric nitrate (Fe(NO3)3) and potassium periodate (KIO3) are generally used. These oxidizers are disclosed in pages 299-300 of the above-cited book (xe2x80x9cThe Science of CMPxe2x80x9d). Of these oxidizers, hydrogen peroxide has become used largely particularly in the recent years because it contain no metallic ion.
The techniques of forming wiring and plugs by CMP using a polishing solution containing the prior art abrasives for polishing metallic film as a main ingredient, however, has the following problems (1) to (7).
(1) There is observed a phenomenon that the superficial central part of metallic wiring inlaid in the grooves formed on the insulating film is excessively polished and concaved as compared with the peripheral parts (hereinafter, this phenomenon is called dishing) and a phenomenon that the superficial part of insulating film surrounding wiring is polished (hereinafter, this phenomenon is called erosion). Dishing and erosion incur an important problem especially in bonding pad and dense line pattern having a large area (about 0.1 mm square) of metallic part. This problem is mentioned in J. Electrochemical Soc. Vol. 141, No. 10, October 1994, pp. 2842-2848.
(2) The abrasive for polishing causes scratches. The scratches are formed not only on the surface of wiring metal layer but also on the surface of insulating film exposed by the CMP method.
(3) The procedure of CMP causes a high frictional force between the polishing abrasive particles and the surface of metal film, due to which delamination can occur between the metal film and the underlying insulating layer or between the SOG (spin on glass) of the underlying insulating layer and the chemical vapor deposition oxide film (hereinafter, referred to as CVD).
(4) A number of abrasive particles remain on the surface of wafer just after being subjected to CMP, which have to be cleaned off prior to the subsequent step until the number of the particles becomes under a predetermined value (for instance, until the number of particles having a size of 0.2 micrometer or above becomes 100 or less per wafer). This can be achieved only by using a cleaning machine carrying out not only a chemical cleaning but also a mechanical cleaning.
Generally, a combination of brush scrub cleaning and megasonic cleaning by the use of a chemical agent is adopted. The brushing material has to be a special material not injuring the surface of metal film. As the chemical agent, aqueous solutions of ammonium hydroxide, hydrofluoric acid, or the like are used, for instance. An example of such a post-CMP cleaning process is described in xe2x80x9cSemiconductor Worldxe2x80x9d May, 1995, page 172.
(5) The consumables used in CMP process are expensive, because production cost of abrasive is high and regulation of its particle size requires an extremely careful work. Particularly, alumina abrasive is several times more expensive than silica abrasive. The cost problem of CMP process is referred to in, for example, xe2x80x9cRecent Topics and Problems in CMP Machines and Consumablesxe2x80x9d (Japanese), Saishin Gijutsu Koza, published by Realize Co., May 1996.
(6) In addition to the above-mentioned problems of CMP apparatus and post-cleaning apparatus, another problem must be referred to with regard to CMP-related apparatus and equipment. It is a problem that CMP requires an apparatus for supplying slurry and an apparatus for disposing the waste slurry and these apparatuses elevate the overall cost of CMP process greatly. Thus, the slurry supplying apparatus must have an agitator for preventing the sedimentation of abrasive and a mechanism for circulating slurry steadily for prevention of its sedimentation in piping. Further, the treatment of waste slurry is costly, and a technique for its reuse is also demanded.
(7) Another problem of CMP is that a CMP apparatus must be installed in a clean room in spite of the large quantity of dust-formable abrasive which CMP uses. Atmosphere of the clean room must be maintained on a high level of cleanliness by attaching a dust-controlling means such as exhaust duct to the CMP apparatus or by providing a special chamber in the clean room, which are both costly.
All the problems mentioned above are caused by the use of an abrasive-containing polishing slurry in the prior CMP method. Nevertheless, the use of abrasive has been indispensably necessary to the prior art CMP method for the purpose of realizing mechanical effect enough to remove the oxide layer formed by oxidizer rapidly. No practical velocity of polishing has been achievable without use of abrasive.
On the other hand, the present inventors have discovered a method for forming an inlaid interconnect structure by polishing a metal film with a polishing solution containing no abrasive, based on their studies up to today. That is, it has been found that an inlaid metal interconnect can be formed by mechanically polishing the surface of a metal film with a polishing solution comprising an oxidizer, a substance which renders an oxide water-soluble, water and, according to the need, a corrosion inhibitor. In an exemplified case of such a method, a copper interconnect is formed by the use of an abrasive-free polishing solution comprising aqueous hydrogen peroxide, citric acid and benzotriazole (hereinafter referred to as BTA).
Although the above-mentioned problems (1) to (7) can be solved by the use of such an abrasive-free polishing solution, there arises in such a case a new problem that the removal rate is lower than in the case of using an abrasive-containing polishing slurry, and especially a problem that the removal rate reaches a saturation and cannot be further elevated even under a high down force of 300 g/cm3 or above. For example, in the case of commercially available alumina slurry, the removal rate can reach a high value of 200-400 nm/minute, even though the problem of scratch becomes more serious under such a condition. Contrariwise, in the case of the abovementioned abrasive-free polishing solution, the removal rate has been as low as about 100 to 150 nm/minute.
The present invention has been accomplished in view of the problems mentioned above, and an object of the present invention is to provide a polishing method and a method for producing semiconductor devices which can realize a high removal rate in the formation of inlaid metallic interconnect.
Thus, the present invention provides a method for polishing metal film which comprises mechanically polishing the surface of metal film with a polishing solution comprising an oxidizer, a substance which renders an oxide water-soluble, a thickener (an agent which makes liquid viscous) and water.
The present invention further provides a method for producing a semiconductor device which comprises mechanically polishing the surface of metal film of a semiconductor substrate prepared by forming a metal film on an insulating film, with a polishing solution comprising an oxidizer, a substance which renders an oxide water-soluble, a thickener and water.